Improving the surface of ferrous articles by complexing agents



United States Patent 3,529,654 IMPROVING THE SURFACE OF FERROUS ARTICLESBY COMPLEXIN G AGENTS Jerome J. Kanter, Palos Park, Ill., assignor toCrane Co., Chicago, 11]., a corporation of Illinois No Drawing. FiledFeb. 15, 1968, Ser. No. 705,614 Int. Cl. B22c 3/00; B22d 27/18 U.S. Cl.164-58 3 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTIONAn article fabricated of ferrous metal has a strong propensity tocorrode unless the metal is extensively alloyed with a non-ferrous metalsuch as chromium, for

example, or the article is provided with a protective coating. A cast orwrought iron, as well as a plain carbon steel, will readily corrode inan atmosphere where oxygen is present unless a protective coating isformed on the surface of the article. Since the use of a stainlesssteel, for example, in numerous applications Where requirements dictatefabrication of an article in iron or steel, is prohibitively expensive,numerous coating and other surface protection methods have beendeveloped.

The most commonly used simple coating for metal articles is paint.Another coating widely used on ferrous metal articles is zinc; i.e.galvanizing. Chrome plating is also employed where a hard and attractivesurface is desirable.

Each of the aforementioned methods necessarily requires performing aseparate step after fabrication of the articles. This adds to productioncosts. Where a metallic coating such as chromium, for example, isapplied, the increased cost factor is substantial. On the other hand,whereas a paint coating is considerably less expensive, it is alsounsuitable in numerous applications of the ferrous article because thearticle is employed in an environment where the paint itself is attackedand readily breached.

In contrast to the known processes where a protective coating isprovided after the metal article is fabricated, methods have beendeveloped for forming an alloy-type coating on the cast metal during thecasting operation. A process of this nature is described in U.S. Pat.No. 1,725,039, where a chromium alloy surface is formed on a ferrousarticle by coating the mold surface with a chromium powder and an oxideformation inhibitor in an organic binder. The metallic chromium alloyswith the molten base metal at the surface of the cast article.

In a broadly similar process, a hard surface coating of tungsten ormolybdenum carbide is formed on a cast base metal by painting the moldcavity with the finely powdered coating material in a binder. Thecarbide coating formed on the surface of the base metal is highlyresistant to wear and certain types of corrosion. This process isdisclosed in U.S. Pat. No. 1,981,403.

These two surface coating processes offer distinct advantages in theformation of protective surfaces on base metal castings in that theprotective coatings are formed during the casting operation. There areother coating or surface treatment processes of this general nature, ofcourse. None provides the ultimate simplicity and minimum cost factorswhich are highly desirable and constantly sought, however.

SUMMARY OF THE INVENTION The present invention is directed to animproved process for forming a hard, durable, corrosion resistantcoating on ferrous articles. More particularly, it relates to thedevelopment of a corrosion resistant surface on cast ferrous articles.It is an object of the invention to provide a process for forming anextremely hard, durable, corrosion resistant coating on a cast ferrousarticle by a simple and inexpensive treatment of the mold prior tocasting the article. A further object of the invention is to provide aprocess wherein painting a mold cavity with a wash containing one ormore compounds of a group consisting of sodium chromate, sodiumaluminate, aluminum hydrate, and the rare earth metal hydrates, prior tocasting, results in a mineral oxide complex of the spinel type beingformed on the surface of the cast ferrous metal articles.

To the accomplishment of the foregoing related ends, the inventioncomprises the features hereinafter clearly described and pointed out inthe claims, the following description setting forth in detail certainillustrative embodiments of the invention. It should be understood,however, that these are indicative of but a few of the various ways inwhich the principles of the invention may be employed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Briefly stated, the presentinvention contemplates painting or daubing a wash containing a compoundfrom a group consisting of sodium chromate, sodium aluminate, aluminumhydrate, and the rare earth metal hydrates, on the cavity surface in amold. The cavity might be formed in a green sand mold or a chill mold,for example. The mold might be baked to a dry sand mold subsequent tothe wash being applied or the wash might be applied directly to a drysand mold.

A molten ferrous base metal is intoduced to the treated (painted ordaubed) mold cavity. As the base metal solidifies in the cavity, thepresence of free oxygen at the inter face between the article and themold causes oxidation to take place. Without prior treatment of the moldaccording to the invention, this oxidation would normally result in theformation of the base metal oxide on the article surface.

The normally formed base metal oxide would, of course, offer someresistance to corrosion. However, it is not a hard, durable surfacewhich is highly resistant to corrosion in high temperature and pressureenvironments.

In the present invention, a more complex oxide is formed from both thebase metal and the metallic elemen-t(s) in the compound(s) washed ontothe surface of the mold cavity prior to casting. The complex oxide is ofthe spinel type. In this light, the spinels are generally defined as agroup of isomorphous mineral oxides having the generic formula AB Owhere A is ferrous iron, and B is one of a group of minerals includingaluminum, chromium, and the rare earth elements.

A spinel type oxide complex is characterized by its gem-like hardness.It constitutes a durable and extremely corrosion resistant coating onthe base metal. The coating, because of its hardness and durability, ishighly corrosion resistant under extremely adverse conditions, includingthose aforementioned conditions where high temperatures and pressuresare encountered and corrosion is normally a serious problem.

The use and application of the process embodying features of theinvention may best be understood by reciting an example; A boilercasting is necessarily exposed to the corrosive and erosive action offluids under high pressures and temperatures on its inner surfaces.Because of the economic factors involved in providing a corrosionresistant coating on the exposed inner surface of such a casting byconventional and well-known techniques, it is customary in boilercastings to rely on the oxide coating normally formed on the surfaceduring casting as a corrosion inhibitor. Unfortunately, although thisoxide coating does provide a certain deterent factor, corrosion stilloccurs at a rapid rate because of the nature of the environment.

To make the inner surface of the boiler casting highly corrosionresistant according to the present invention, a wash is preparedcomprising a solution of sodium chromate (Na CRO for example, in water.At the same time, a conventional green sand mold, for example, isprepared. The wash is painted or daubed on the surface of the moldcavity where a corrosion resistant surface is desired on the articlebeing cast. The green sand mold is then baked at approximately 400 F.600F. to form a dry sand mold with the sodium chromate present on the moldcavity surfaces which have been treated with the wash.

The mold having been prepared in the aforedescribed manner, moltenferrous iron is introduced to the mold cavity by conventionaltechniques. The molten iron solidifies in the mold to form the articlebeing cast; in the present instance, a boiler casting. The normalpresence of oxygen in the sand at the interface between the solidifyingarticle and the mold surface causes oxidation of the casting base metalto take place on the casing surface.

In the present invention this oxidation process is more complex thanmerely a formation of base metal oxide. As the ferrous iron oxidizes,the chromium in the sodium chromate is taken into the oxidationreaction. A new and complex oxide of the spinel type is formed on thesurface of the article by the oxidation process. In the present example,this compound comprises iron chromium oxide or FeCr O The spinel-likeoxide complex which forms on the surface of the cast ferrous articlehas, in a characteristic manner, a gem-like hard surface. This surfaceis extremely durable and corrosion resistant under the substantiallyadverse conditions to which a boiler casting inner surface is subjected;i.e. high pressures and temperatures.

In the example utilized, the mold has been described as a green sandmold which is baked to a dry sand mold prior to casting and after thewash is applied. It should be reiterated here that the invention alsocontemplates applying the wash to a chill mold, for example, or a drysand mold after baking. It is further within the purview of theinvention that the wash be applied and the invention employed withvarious other types of molds.

In addition, the exemplary wash employed is an aqueous solution ofsodium ch-romate. The wash might, however, comprise a sodium aluminate,aluminum hydrate, or rare earth metal hydrate compound in an aqueoussolution. In any such case a hard, durable, corrosion resistant surfaceof the nature contemplated is formed.

In this light, although the resultant hard, durable, corrosion resistantsurface which forms on the ferrous metal casting according to theinvention has been described as a spinel-type oxide complex, it ispossible that the hard coating might comprise other elements or more orless complex compounds. Such an eventuality and resultant surfacecoating make-up are also within the contemplation of the invention.

Regardless, the corrosion resistant surface which is formed on thesurface of a cast metal article such as a boiler casting or the like bythe process embodying features of the invention is highly effectiveunder the most adverse conditions. Nevertheless, the process itself isexceedingly simple and inexpensive. Cast ferrous articles can be madewith highly corrosion resistant surfaces at costs compatible with theirapplications.

While the embodiment described herein is at present considered to bepreferred, it is understood that various modifications and improvementsmay be made therein, and it is intended to cover in the appended claimsall such modifications and improvements as fall within the true spiritand scope of the invention.

What is desired to be claimed and secured by Letters Patent of theUnited States is:

1. A process for forming a protective coating on a cast ferrous articlecomprising the steps of:

(a) treating a surface of a mold cavity with a compound or compoundsfrom a group consisting of sodium aluminate, sodium ch-romate, aluminumhydrate, and the rare earth metal hydrates,

(b) and introducing molten ferrous metal to the cavity so that it formsa casting which comes into contact with said compound at said surface,

(c) whereby a chemical complex coating is formed on the casting and saidcomplex coating has a hard, corrosion resistant surface.

2. The process of claim 1 further characterized in that:

(a) said compound is coated on said mold cavity by applying a liquidwash containing the compound to the walls of the cavity.

3. The process of claim 1 further characterized in that:

.(a) said mold cavity is formed in a green sand mold,

(b) said mold cavity is coated with said compound by preparing a liquidwash containing said compound and coating the cavity with said wash,

(c) said green sand mold being baked for a predetermined length of timeafter the application of said wash and before introducing the moltenferrous metal to the cavity to form a dry sand mold.

References Cited UNITED STATES PATENTS 1,166,167 '12/1915 Woods 164-582,847,741 8/1958 Meves et a1. 16414 3,121,269 2/1964 Nefi' 16414 1.SPENCER OVERHOLSER, Primary Examiner J. E. ROETHEL, Assistant ExaminerUS. Cl. X.-R.

